1. Field of the Invention
The invention relates to a coking drum, to a shut-off valve for a coking drum, and to a method of feeding residual materials into a coking drum.
2. Description of the Related Art
In crude oil processing, importance is increasingly being placed on the further processing of heavy products left over from the distillation of crude oil to lighter products. Accordingly, the production of heavy heating oil is to be reduced and the production of gasoline, diesel fuel and light heating oil increased without the need to process additional crude oil. The plants required for that purpose, which operate according to various methods, are called conversion plants. They convert heavy, long hydrocarbon molecules into light, shorter hydrocarbon molecules by cracking the long molecules. A distinction is made between three such cracking methods: thermal cracking, catalytic cracking and hydrocracking. The optimum combination of methods depends on several factors which include, inter alia, the quality of the crude oil in question and the products desired. The various cracking methods are based on different feed products, gas oil from vacuum distillation acting as the feed product for catalytic cracking plants and hydrocrackers, and the residue from vacuum distillation being used as feed product in visbreakers or cokers.
The longest known and probably simplest cracking method is thermal cracking. In that method, hydrocarbon chains are cracked at high temperatures. The group of thermal cracking methods includes so-called visbreaking and coking in which carbon in solid form, so-called coke or petroleum coke, is deposited.
A distinction is made between three different methods of coking, namely so-called fluid coking, so-called delayed coking and so-called flexicoking. The most frequently used method is that of delayed coking. In the method, the feed product which, for example, may be the residue from vacuum distillation, is introduced into a furnace at a pressure of about 30 bar and heated to about 500° C. As a result of those conditions, it flows through the furnace at very high speed and then cokes when admitted into a coking chamber or coking drum having a prevailing chamber pressure of about 4 bar that is separate from the furnace and connected thereto by a pipeline.
There are generally associated with each furnace at least two coking drums, one or other of those chambers being in operation while coke is being removed from the other one. The coke can be cut out of the coking drum by means of, for example, water under high pressure. The light hydrocarbons produced during the coking process are conveyed from the coking chamber into a fractionating tower where they are further processed. The resulting petroleum coke is called green coke and, after comminution, can either be sold or further refined. Further refining takes place in a calcination process in which, at temperatures of 1200° C. and above, any oil constituents still present are burnt off and coked. The calcination product obtained in that process can then be utilized, for example, for electrodes which are used in the aluminum industry.
The particular drum that has been filled is cooled, and the coke is removed from it. For that purpose, water is first introduced into the drum in order to cool the hot coke obtained. The drum is then opened to the atmosphere by opening, depending on the design of the coking chamber or drum, either only the bottom end of the drum or also the top end of the drum or chamber, so that the coke can be cut out of the drum and delivered for further use.
The operation of opening the coking drum may present sources of risk for several reasons. The water introduced into the drum for cooling the coke prior to the drum being opened is very hot and, if the equipment is not handled carefully, may result in injuries caused by emerging hot water or steam, which is under pressure. In addition, loose pieces of coke may fall out of the drum or place such a strain on the opening mechanisms that, once they have been unlocked, they open abruptly, which may also result in injuries to the operating personnel. The operating personnel may also be exposed to dusts, especially coke particles, and also to irritant or toxic gases when the drums are opened.
In order to reduce the sources of risk to the operating personnel, a change was made from opening the coking drums manually to opening them automatically by means of dedicated, for example hydraulic, shut-off valves. Such a coking drum having a shut-off valve, which is in the form of a bridging-pipe gate valve, is known from EP 1 516 908 B1. The apparatus described in that publication is a coking drum in which there is arranged at an outlet and/or at an inlet a shut-off member in the form of a bridging-pipe gate valve, having two shut-off plates. Also, residual materials, especially from a vacuum distillation process upstream of the coking, are there fed into the coking drum through a feed opening which is provided at some distance above the shut-off valve in a side wall of the coking drum.
A drawback of that known coking drum is that a homogeneous temperature distribution and a uniform distribution of residual materials in the coking drum is not ensured.
The problem of the present invention is accordingly to provide a coking drum and shut-off valve and also a method which ensure improved feeding of residual materials into the coking drum so that, especially, a homogeneous temperature distribution is ensured in the coking drum during the coking process.